1. Field of the Invention
The present invention generally relates to a signal transmission system for transmitting serially the signals generated in parallel. More specifically, the present invention is concerned with a sensor signal transmission system for transmitting serially to a central control station sensor signals generated in parallel by sensors installed at locations remote from the central control station for monitoring or supervising operations or states of devices, instruments, machines, electric or mechanical elements or the like which are to be monitored by the associated sensors, respectively.
2. Description of the Related Art
In the field of the automatic control techniques, there is employed widely such a signal transmission system in which control signals are transmitted from a central control station including a sequence controller, programmable controller, a computer or the like to a number of controlled devices such as electric motors, solenoids, electromagnetic valves, relays, thyristors, lamps and/or the like installed at locations remotely from the central control station for controlling the operations thereof, and in which the sensor signals indicating the states of the controlled devices (such as on/off state, positional displacement, angular positions, temperatures or the like) detected by sensor means such as reed switches, micro-switches, electronic switches, photoelectric detectors or the like bi-state (on/off-state) sensor elements are transmitted to the central control station for evaluation of the operating states of the controlled devices.
In conjunction with the transmission systems of the type mentioned above, there are a lot of such applications where the devices to be controlled are miniaturized and provided in a great number, being arrayed densely to one another, as a result of which a great difficulty is encountered in making access to the individual devices. Consequently, provision of individual control signal lines, clock signal lines, power supply lines and others between the control station and the devices to be controlled involves much labor, large space and high costs. Besides, maintenance is attended with very troublesome procedure.
As the typical examples of the device to be controlled, there may be mentioned an automated tool such as, for example, an industrial robot which is designed to be hydraulically or pneumatically controlled with the aid of electromagnetic valves also referred to as the solenoid valve. In most of such automated tools or robots, a so-called manifold solenoid valves unit incorporating integrally a number of solenoid valves is used for controlling hydraulically or pneumatically various parts of the tool with a view to reducing the space occupied by the valves. However, in order to remotely control the individual solenoid valves realized in the form of the manifold valve unit from a control station, a number of lines inclusive of the control signal lines, the power supply line and others have to be wired between the control station and the individual solenoid valves to be controlled, respectively, which of course requires high expenditure as well as a large space.
As an attempt for solving the problems of the prior art control/supervisory signal transmission systems such as described above, there can be mentioned a technique which is disclosed in U.S. patent application Ser. No. 237,387 filed under the title "REMOTE CONTROL SYSTEM OF SERIAL/PARALLEL CONVERSION TYPE" on Aug. 26, 1988 (now matured to U.S. Pat. No. 4,937,568 issued Jun. 26, 1990) and which is assigned to the assignees of the present application, the whole disclosure of which is herein incorporated by reference.
For having a better understanding of the invention, the remote control system disclosed in U.S. patent application Ser. No. 237,387 (U.S. Pat. No. 4,937,568) will be described by reference to FIGS. 7A and 7B of the accompanying drawings.
Referring to FIG. 7A, a reference numeral 10 denotes a central control station and a numeral 11 generally denotes a local station which comprises a start bit unit 12 and a conversion unit 13. In the central control station 10, data indicating a control command is externally inputted from a controller such as a sequence controller (not shown) to a parallel-to-serial (parallel/serial) conversion circuit 101 in the form of parallel data bits through an appropriate input unit (not shown). The parallel/serial conversion circuit 101 converts the input data bits into serial signal pulses under the timing commanded by a clock signal generated by a clock generating circuit 102. The serial signal pulses as generated are inputted to a signal conversion circuit 103 together with the clock pulses. The signal conversion circuit 103 serves for the function to superpose the serial data or signal pulses and the clock pulses on a D.C. power. The D.C. power superposed with the serial signal pulses and the clock pulses and outputted from the signal conversion circuit 103 is then sent out onto a line 104 as a serial output signal "OUT" which has such a waveform as shown in FIG. 7B at (a).
Additionally, the signal conversion circuit 103 is so designed as to generate a start signal "START" in synchronism with the start of the pulse train superposed as mentioned above. Refer to FIG. 7B at (b). The start signal is sent out onto a line 105 labeled "START". Incidentally, a reference numeral 106 denotes a ground potential line (GND).
The D.C. power outputted from the signal conversion circuit 103 thus assumes such a waveform as illustrated in FIG. 7B at (a). More specifically, a level V.sub.x represents the voltage level of the D.C. power (in volts), V.sub.x/2 represents the voltage level corresponding to the command or control signal pulse of logic "0" level, and 0 (zero) represents the voltage level (zero volt) corresponding to the command or control signal pulse of logic "1", wherein the signal pulses of logic "1" and "0" levels are in synchronism with the clock pulses, respectively.
Upon reception of the pulse-superposed voltage "OUT" by the local station 11 via the line 104, a load driving power restoration circuit (PRC) 122 regenerates a power having a voltage level substantially equal to the level V.sub.x for energizing or driving devices or loads connected to output circuits 138, 139 by eliminating the pulse components from the input pulse-superposed voltage. The input pulse-superposed voltage is also applied to stabilized constant voltage power generating circuits or voltage converters (CVC) 121 and 131, whereby a constant voltage (having a level lower than V.sub.x) is generated to be supplied as the source voltage to various constituent circuits of the local station, all of which are constituted by electronic circuits of low power consumption type. The output of the load driving power restoration circuit 122 is connected to a line V.sub.d which in turn is connected to power input terminals of the output circuits 138 and 139 to which the devices or loads (not shown) to be controlled are connected. The power line V.sub.d may additionally be connected to a terminal 124 of a D.C. power supply source for emergency so that the loads can be operated even when the power supply from the central control station 10 should be interrupted for some reason. A start signal detecting circuit 123 constituting a part of the start bit unit 12 and energized by the constant voltage power supply circuit (CVC) 121 detects the start signal st supplied via the signal line 105 in synchronism with a first pulse t.sub.1 of logic "1" (see FIG. 7B at (b)). The detected start pulse st is supplied to a signal distribution circuit 133. On the other hand, a signal extracting circuit 132 connected to the power line 104 detects the superposed data signal pulses discriminatively with regard to the pulse levels to thereby output the clock pulses ck and data signal pulses of logic level "1" and "0" designated generally by dt.
The clock pulse ck is supplied to the signal distributing circuit 133 to allow the logic "1" pulse of the start signal st outputted from the start signal detecting circuit 123 to be inputted to the clock pulse distributing circuit 133, resulting in that the pulse of logic level "1" is produced from the output terminal Q1 of a first stage of the signal distribution circuit 133 to be applied to a clock input terminal CP of a latch circuit 134.
Thus, at the time t.sub.1 when the leading clock pulse ck makes appearance, the first data pulse of logic "1" (pulse t.sub.1 shown in FIG. 7B at (a)) inputted to a control pulse input terminal D of the latch circuit 134 is latched by the latter. As a result, an output signal is produced from an output terminal Q of the latch circuit 134 to thereby turn on the associated output circuit 138 which may be constituted by a switch. Consequently, the electric power generated by the power restoration circuit (PRC) 122 is supplied to the device to be controlled and connected to the output circuit 138 for electrically energizing the device, which may be a solenoid of an electromagnetic valve, an electric motor, a relay or the like, although not shown.
The pulse making appearance on the line 104 at a time point t.sub.2 in the pulse train illustrated in FIG. 7B at (a) is logic "0". Consequently, the signal extracting circuit 132 produces as the output thereof the clock pulse ck and the data signal pulse dt of logic "0". The clock pulse ck is applied to the signal distributing circuit 133, as the result of which the data of logic "1" set at the first stage of the signal distributing circuit 133 at the preceding time point t.sub.1 is shifted to a second stage of the circuit 133, whereby the data signal pulse of logic "1" is generated at an output terminal Q2 to be applied to the clock input terminal CP of a latch circuit 135. This results in that the signal pulse of logic "0" outputted from the signal extracting circuit 132 is latched and held by the latch circuit 135. At this time, no output signal is produced from the output terminal Q of the latch circuit 135. Accordingly, the output circuit 139 remains inoperative.
Simultaneously with the output of the clock pulse from the output terminal Q2 of the signal distributing circuit 133, a succeeding stage start signal generating circuit 137 is driven in response to the signal appearing at an output terminal Q2 of the signal distributing circuit 133, whereby the start signal is supplied to the succeeding conversion unit.
As will now be appreciated from the above description, the control data pulse train transmitted serially via the transmission line 104 undergoes serial/parallel conversion in the conversion unit 13 of the local station 11, whereby the output circuits 138 and 139 connected to the output side of the conversion unit 13 as well as those of the succeeding conversion unit (not shown in FIG. 7A) are set to the states of "ON", "OFF", "ON", "OFF" and "OFF", respectively, in response to the control data pulse train illustrated in FIG. 7B at (a) on the assumption that three output circuits and three latch circuits are provided in the succeeding conversion unit with the signal distribution circuit being constituted in three stages. The abovementioned state is held as it is until the next control data pulse train is issued from the central control station 10.
As is apparent from the above, the subject matter of U.S. patent application Ser. No. 237,387 (U.S. Pat. No. 4,937,568) is directed to the transmission system for transmitting the control signal from a central control station to the remotely located devices to be controlled. With the arrangement described above, the number of lines for transmission of various control signals and power can be significantly decreased. In other words, the various devices provided at remote stations can be controlled with a significantly reduced amount of wiring hardware. However, it is noted that the technique disclosed in U.S. patent application Ser. No. 237,387 (U.S. Pat. No. 4,937,568) is concerned only with the transmission of the control signals to the devices installed at remote locations through a power transmission line, and no consideration is paid to the transmission of sensor signals from the local stations to the central control station for the purpose of monitoring or supervising the operating states of the devices controlled at the central station side. In general, in the remote control system to which application of the technique disclosed in the recited reference is intended, it is indispensably required to monitor the states of the devices to be controlled with the aid of appropriate types of sensors, since otherwise the desired control can not be performed in a satisfactory manner. Thus, there exists a need for transmission of the sensor signals from the local stations to the central control stations for the purpose of evaluation of the operating states of the devices controlled. In this conjunction, transmission of the sensor signals should also be realized with a minimum number of lines (i.e. with a minimum amount of wiring hardware).